Doug Gregor has indicated to me that he is not able to continue with the development/maintenance of Boost.Signals.
[ This is not a surprise, the last significant change to the library was back in 2004. ]
Fortunately, Frank Hess' Signals2 library is going strong, and he has a porting guide for people currently using Boost.Signals at: http://www.boost.org/doc/html/signals2/api_changes.html#signals2.porting
For the upcoming 1.54 release, I have added notes to this effect to the Boost.Signals documentation, and added a warning to